i've used olive oil, canola oil, butter, baby oil, body moisturiser, vaseline, and water based lube like astro glide.
olive oil is my favorite, but it's hard to clean and if if gets on anything can ruin it.
there is nothing better than oiling up a guys cock with olive oil and feeling him go hard as you rub it over his cock and balls. wait, yes there is ... having him oil up your cock at the same time.
being olive oil of course you can suck it off his cock afterwards too.